001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.base;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import com.google.common.annotations.Beta;
022import com.google.common.annotations.GwtCompatible;
023
024import java.io.Serializable;
025import java.util.Iterator;
026import java.util.Set;
027
028import javax.annotation.Nullable;
029
030/**
031 * An immutable object that may contain a non-null reference to another object. Each
032 * instance of this type either contains a non-null reference, or contains nothing (in
033 * which case we say that the reference is "absent"); it is never said to "contain {@code
034 * null}".
035 *
036 * <p>A non-null {@code Optional<T>} reference can be used as a replacement for a nullable
037 * {@code T} reference. It allows you to represent "a {@code T} that must be present" and
038 * a "a {@code T} that might be absent" as two distinct types in your program, which can
039 * aid clarity.
040 *
041 * <p>Some uses of this class include
042 *
043 * <ul>
044 * <li>As a method return type, as an alternative to returning {@code null} to indicate
045 *     that no value was available
046 * <li>To distinguish between "unknown" (for example, not present in a map) and "known to
047 *     have no value" (present in the map, with value {@code Optional.absent()})
048 * <li>To wrap nullable references for storage in a collection that does not support
049 *     {@code null} (though there are
050 *     <a href="http://code.google.com/p/guava-libraries/wiki/LivingWithNullHostileCollections">
051 *     several other approaches to this</a> that should be considered first)
052 * </ul>
053 *
054 * <p>A common alternative to using this class is to find or create a suitable
055 * <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">null object</a> for the
056 * type in question.
057 *
058 * <p>This class is not intended as a direct analogue of any existing "option" or "maybe"
059 * construct from other programming environments, though it may bear some similarities.
060 *
061 * <p>See the Guava User Guide article on <a
062 * href="http://code.google.com/p/guava-libraries/wiki/UsingAndAvoidingNullExplained#Optional">
063 * using {@code Optional}</a>.
064 *
065 * @param <T> the type of instance that can be contained. {@code Optional} is naturally
066 *     covariant on this type, so it is safe to cast an {@code Optional<T>} to {@code
067 *     Optional<S>} for any supertype {@code S} of {@code T}.
068 * @author Kurt Alfred Kluever
069 * @author Kevin Bourrillion
070 * @since 10.0
071 */
072@GwtCompatible(serializable = true)
073public abstract class Optional<T> implements Serializable {
074  /**
075   * Returns an {@code Optional} instance with no contained reference.
076   */
077  public static <T> Optional<T> absent() {
078    return Absent.withType();
079  }
080
081  /**
082   * Returns an {@code Optional} instance containing the given non-null reference.
083   */
084  public static <T> Optional<T> of(T reference) {
085    return new Present<T>(checkNotNull(reference));
086  }
087
088  /**
089   * If {@code nullableReference} is non-null, returns an {@code Optional} instance containing that
090   * reference; otherwise returns {@link Optional#absent}.
091   */
092  public static <T> Optional<T> fromNullable(@Nullable T nullableReference) {
093    return (nullableReference == null)
094        ? Optional.<T>absent()
095        : new Present<T>(nullableReference);
096  }
097
098  Optional() {}
099
100  /**
101   * Returns {@code true} if this holder contains a (non-null) instance.
102   */
103  public abstract boolean isPresent();
104
105  /**
106   * Returns the contained instance, which must be present. If the instance might be
107   * absent, use {@link #or(Object)} or {@link #orNull} instead.
108   *
109   * @throws IllegalStateException if the instance is absent ({@link #isPresent} returns
110   *     {@code false})
111   */
112  public abstract T get();
113
114  /**
115   * Returns the contained instance if it is present; {@code defaultValue} otherwise. If
116   * no default value should be required because the instance is known to be present, use
117   * {@link #get()} instead. For a default value of {@code null}, use {@link #orNull}.
118   *
119   * <p>Note about generics: The signature {@code public T or(T defaultValue)} is overly
120   * restrictive. However, the ideal signature, {@code public <S super T> S or(S)}, is not legal
121   * Java. As a result, some sensible operations involving subtypes are compile errors:
122   * <pre>   {@code
123   *
124   *   Optional<Integer> optionalInt = getSomeOptionalInt();
125   *   Number value = optionalInt.or(0.5); // error
126   *
127   *   FluentIterable<? extends Number> numbers = getSomeNumbers();
128   *   Optional<? extends Number> first = numbers.first();
129   *   Number value = first.or(0.5); // error}</pre>
130   *
131   * <p>As a workaround, it is always safe to cast an {@code Optional<? extends T>} to {@code
132   * Optional<T>}. Casting either of the above example {@code Optional} instances to {@code
133   * Optional<Number>} (where {@code Number} is the desired output type) solves the problem:
134   * <pre>   {@code
135   *
136   *   Optional<Number> optionalInt = (Optional) getSomeOptionalInt();
137   *   Number value = optionalInt.or(0.5); // fine
138   *
139   *   FluentIterable<? extends Number> numbers = getSomeNumbers();
140   *   Optional<Number> first = (Optional) numbers.first();
141   *   Number value = first.or(0.5); // fine}</pre>
142   */
143  public abstract T or(T defaultValue);
144
145  /**
146   * Returns this {@code Optional} if it has a value present; {@code secondChoice}
147   * otherwise.
148   */
149  public abstract Optional<T> or(Optional<? extends T> secondChoice);
150
151  /**
152   * Returns the contained instance if it is present; {@code supplier.get()} otherwise. If the
153   * supplier returns {@code null}, a {@link NullPointerException} is thrown.
154   *
155   * @throws NullPointerException if the supplier returns {@code null}
156   */
157  @Beta
158  public abstract T or(Supplier<? extends T> supplier);
159
160  /**
161   * Returns the contained instance if it is present; {@code null} otherwise. If the
162   * instance is known to be present, use {@link #get()} instead.
163   */
164  @Nullable
165  public abstract T orNull();
166
167  /**
168   * Returns an immutable singleton {@link Set} whose only element is the contained instance
169   * if it is present; an empty immutable {@link Set} otherwise.
170   *
171   * @since 11.0
172   */
173  public abstract Set<T> asSet();
174
175  /**
176   * If the instance is present, it is transformed with the given {@link Function}; otherwise,
177   * {@link Optional#absent} is returned. If the function returns {@code null}, a
178   * {@link NullPointerException} is thrown.
179   *
180   * @throws NullPointerException if the function returns {@code null}
181   *
182   * @since 12.0
183   */
184  public abstract <V> Optional<V> transform(Function<? super T, V> function);
185
186  /**
187   * Returns {@code true} if {@code object} is an {@code Optional} instance, and either
188   * the contained references are {@linkplain Object#equals equal} to each other or both
189   * are absent. Note that {@code Optional} instances of differing parameterized types can
190   * be equal.
191   */
192  @Override
193  public abstract boolean equals(@Nullable Object object);
194
195  /**
196   * Returns a hash code for this instance.
197   */
198  @Override
199  public abstract int hashCode();
200
201  /**
202   * Returns a string representation for this instance. The form of this string
203   * representation is unspecified.
204   */
205  @Override
206  public abstract String toString();
207
208  /**
209   * Returns the value of each present instance from the supplied {@code optionals}, in order,
210   * skipping over occurrences of {@link Optional#absent}. Iterators are unmodifiable and are
211   * evaluated lazily.
212   *
213   * @since 11.0 (generics widened in 13.0)
214   */
215  @Beta
216  public static <T> Iterable<T> presentInstances(
217      final Iterable<? extends Optional<? extends T>> optionals) {
218    checkNotNull(optionals);
219    return new Iterable<T>() {
220      @Override
221      public Iterator<T> iterator() {
222        return new AbstractIterator<T>() {
223          private final Iterator<? extends Optional<? extends T>> iterator =
224              checkNotNull(optionals.iterator());
225
226          @Override
227          protected T computeNext() {
228            while (iterator.hasNext()) {
229              Optional<? extends T> optional = iterator.next();
230              if (optional.isPresent()) {
231                return optional.get();
232              }
233            }
234            return endOfData();
235          }
236        };
237      }
238    };
239  }
240
241  private static final long serialVersionUID = 0;
242}